In the field of well logging, there are several well-known methods for displacing measuring instruments in both greatly inclined and horizontal pipes. It is possible to use drill rods made up end to end and equipped with a transmission cable placed in the inner space thereof, a continuous steel tubing (cabled coiled tubing) also comprising a transmission cable, or mechanical means for displacing measuring tools, such as hydraulically, electrically or electro-hydraulically-powered tractors. Measuring sonde tractors are generally fed by an electric cable which supplies electric power to an electric motor driving a high-pressure hydraulic pump. The high-pressure hydraulic fluid actuates wheels placed against the wall of the well or of the pipe. The tensile or thrust force is of the order of 5 to 10 kN. These tractors are mechanically complex and they are not currently available with small outside diameters. For example, in the case of the most powerful tractors on the market, the latter cannot be fed into pipes having an inside diameter below 6 inches (152 mm).
In a certain number of well patterns, the cased pipe is extended by a drain hole drilled by means of a tool whose diameter is smaller than the inside diameter of the cased pipe. The tractor used for maneuvering instruments or equipments in the wellbore cannot enter the drain hole.